The present invention generally relates to an image signal processing device, and particularly to the setting of a reference which is used when an analog image signal is quantized to generate a digital image signal. The present invention is suitable for an image reading apparatus such as a scanner and a facsimile machine.
Currently, an image reading apparatus such as a scanner and a facsimile machine is widely used. In such an apparatus, an analog image signal is generated from a light reflected on a document and is then converted into binary-valued or multi-valued image data.
FIG. 1 is a schematic block diagram of a conventional image processing device provided in a scanner. Referring to FIG. 1, a scanner 1 includes a line image sensor 2, an amplifier 3, a white peak level holding circuit 4, a timing circuit 5, a driving circuit 6, a voltage dividing circuit 7, and a comparator 8. The line image sensor 2 has a full-size charge coupled device (hereinafter simply referred to as a CCD device) amounting to one line, and divides an electrical signal corresponding to a light reflected on a document into parts for every pixel. The divided signals form an analog image signal and is supplied to the amplifier 3. The line image sensor 2 is driven by the driving circuit 6. The read timing is defined for every pixel by a timing signal derived from the timing circuit 5.
The amplifier 3 amplifies the image signal supplied from the line image sensor 2, and supplies the white peak level holding circuit 4 and the comparator 8 with an amplified image signal. The white peak level holding circuit 4 operates in synchronism to the timing signal supplied from the timing circuit 5, and holds a peak level of an image signal obtained when reading a white press plate used for supporting a document set on a contact glass of the scanner.
Referring to FIG. 2, there is illustrated an example of the structure for the white peak level holding circuit 4. As is illustrated, the white peak level holding circuit 4 includes amplifiers 11 and 12, a diode 13, a transistor 14, a capacitor 15, and resistors 16 and 17. The transistor 14 is supplied with a timing signal for reset (a reset signal), which is derived from the timing circuit 5. The white peak level holding circuit 4 is reset before the white press plate is read. Thereafter, the image signal supplied from the line image sensor 2 through the amplifier 3 is applied to the amplifier 11. Then, the peak level obtained when a white pixel is read, that is, a peak voltage level is held during a time depending on the time constant defined by the capacitor 15 and the resistor 16. Generally, it is very difficult to hold the peak level for a long time. Normally, the holding time is set to a reading time it takes to read a few lines to one page of a document. It is noted that when the holding time is set equal to a reading time equal to a few lines, it is necessary to hold the white peak level again by utilizing a background portion of the document.
The white peak level held by the white peak level holding circuit is voltage-divided by the voltage dividing circuit 7, which thereby outputs a reference value to be compared in the comparator 8 with the image signal supplied from the amplifier 3. The comparator 8 determines whether the divided image signal is white or black, and outputs binary-valued data based on the comparison results.
In case where a halftone processing is carried out, the voltage dividing circuit 7 generates a plurality of reference values (dither value) arranged in a matrix form on the basis of the white peak level supplied from the white peak level holding circuit 4. Then the comparator 8 compares the image signal with the plurality of reference values, and outputs binary valued data indicating white or black for every pixel.
However, the aforementioned conventional image signal processing device presents the following disadvantages. Since the white peak level is held in the form of an analog signal, the white peak level varies with time and thereby the quality of reproduced images is deteriorated. That is, as shown in FIG. 3(b), the held white peak level gradually decreases, because the white peak level is held during the holding time based on the time constant of the capacitor 15 and the resistor 16. As a result, the reference value determined by dividing the white peak level also decreases as indicated by a broken line shown in FIG. 3(a). This affects the white/black discrimination and causes errors in the discrimination results. It follows that the image quality is deteriorated.
Particularly, in a facsimile machine, the scanner of the transmitter must be stopped for a long time when transmission errors are caused due to a fault occurring in a transmission line and image data is being retransmitted. Also, in transmission of halftone image data, the scanner is required to be stopped for a long time, because the efficiency of data compression is poor. In the above-mentioned cases, the held white peak level decreases during the time when the scanner is stopped. As a result, the held white peak level decreases stepwise as shown in FIG. 3(c). The above decrease of the held white peak level causes an abrupt decrease of the reference value to be compared with the image signal supplied from the amplifier 3. The decrease of the reference value deteriorates the quality of reproduced images.